Modeling the heterogeneous oxidation of elemental mercury by chlorine in flue gas

The chloride (Cl) addition is an effective mercury oxidation technology. However, it is difficult to determine the optimal amount of Cl addition due to the unanswered question of how elemental mercury (Hg-0) is oxidized by Cl. This study used the one-dimensional drop furnace and heterogeneous oxidation models to study the reaction mechanism between Hg-0 and Cl in the combustion flue gas. This study finds that the Cl and Hg-0 oxidation mechanism follows the E-R mechanism. The formed active Cl atom (Cl*) on the active sites directly oxidizes the Hg-0 into the Hg2Cl2 or HgCl2. Then the Hg2Cl2 or HgCl2 transfer from active sites into the gas phase. However, when the addition amount exceeds optimum point, the subsequent Cl addition will not further increase the oxidation ratio of Hg-0. The optimum point of Cl addition is 40 ppm via solution impregnation addition but it is 400 ppm via HCl gas addition, respectively. Therefore, the solution impregnation is a more effective method to oxidize Hg-0 compared with the HCl addition in flue gas. These results provide important guidance on the Cl addition practice in the combustion flue gas.